1. Field of the Invention
The invention relates to piezoelectric microcantilevers for use in atomic force microscopy. More specifically, the invention relates to high sensitivity piezoelectric microcantilevers capable of both static contact imaging and dynamic non-contact imaging.
2. Description of the Related Technology
Atomic force microcopy (AFM) is a method for imaging a specimen with nanoscale resolution. AFM generally utilizes a piezoelectric actuator, deflection sensor and microcantilever probe to detect forces between a specimen and a microcantilever probe, such as mechanical contact forces, van der Waals forces, capillary forces, chemical bonding, electrostatic forces, magnetic forces, etc. The movement of the probe is measured by an optical system by measuring the displacement of the probe. A microcantilever probe is capable of generating a high-resolution image of specimen on an atomic scale.
AFM has two standard operational modes: static contact imaging and dynamic non-contact imaging. In static contact imaging, the tip maintains constant contact with a specimen during scanning. As the tip encounters topographical features of the surface, the microcantilever is deflected or bent. The microcantilever induces a change in voltage responsive to these deflections, and a feedback system controls the distance between the microcantilever and the surface of the specimen so as to maintain a relatively constant force between the tip and specimen surface. In the feedback system, a signal representative of the deflection of the microcantilever is compared against a reference voltage to produce an error signal. Using the feedback electronics to maintain the error signal at or near zero, an output is generated which both maintains the error signal at or near zero by changing the tip-sample spacing, and generates a graphic representation of the surface of the specimen.
Static contact imaging may also involve intermediate tapping of the specimen surface. In a gaseous medium, a microcantilever is oscillated at its resonant frequency and positioned above the surface so that it only taps the surface during a fraction of its oscillation period. Lateral forces are therefore significantly reduced as the tip scans the specimen surface. Tapping is particularly useful when imaging poorly immobilized or soft samples. Tapping may also be applied to a specimen located in a liquid medium, such as in situ DNA, in order to reduce van der Waals forces and eliminate capillary forces between a microcantilever tip and a specimen.
In dynamic non-contact imaging, the cantilever is externally oscillated above a specimen surface at or close to its resonant frequency. The amplitude of oscillation, phase and resonance frequency are modified by the force interaction between the microcantilever probe and specimen. As the microcantilever scans the specimen, the distance between the probe and specimen surface features varies, causing a change in the force gradient. Resultant changes in amplitude of oscillation, frequency or phase of the microcantilever are detected, and a feedback system maintains a substantially constant separation between the microcantilever probe and specimen.
The materials and methods for fabricating piezoelectric microcantilevers significantly influence detection sensitivity and the capability of operating in different AFM imaging modes. Microcantilever probes have previously been constructed from materials having piezoelectric and ferroelectric properties such as Si, Si3N4, ZnO and lead-zirconate-titanate (PZT). Of these, bulk PZT, which has a piezoelectric constant of −d31=320 pm/V, dielectric constant of about 3800 and thicknesses of ≧127 μm, (See www.piezo.com/prodsheet2sq5H.html) offers the most promising properties, but, due to its thickness, is not suitable for use in microcantilevers.
Since resonance sensitivity is inversely proportional to the thickness of the piezoelectric layer of a microcantilever, a current trend in the AFM field is the development of highly sensitive thin film piezoelectric microcantilevers capable of mimicking or outperforming ZnO wires or PZT films. Reducing the microcantilever size, however, diminishes resonance-peak height, rendering the cantilever incapable of high sensitivity detection. Previous strategies for improving PEMS resonance peak height have included amplifying the piezoelectric voltage and reducing the noise with a bridge circuit, incorporation of a piezoelectric patch of the same material and dimension as the PEMS on a silicon substrate, and/or replacing ZnO wires, having piezoelectric coefficients in the range of −d31=−4 pm/V and d33=12.4 pm/V, with PZT films.
Most PZT films exhibit piezoelectric coefficients ranging from −d31=58 pm/V20 to d33=190 to 250 pm/V. Although these values are much higher than those of ZnO, they are only about 20 to 40% of commercial bulk PZT, due to problems with interfacial diffusion and substrate pinning. Prior art methods have been unable to effectively achieve thin self-actuating and self-detecting PEMS having a level of sensitivity comparable to bulk PZT. Thus far, it has not been possible to construct a highly sensitive microcantilever by simultaneously simplifying and miniaturizing detection and actuation elements without sacrificing AFM performance and imaging resolution.
In an attempt to create a more sensitive microcantilever, researchers have developed PZT thin film cantilevers having a piezoelectric constant of −34.2 pm/V, 37% of bulk PZT, and sensitivity of 2.0×10−2 mV/nm (Kanda et al., “A flat type touch probe sensor using PZT thin film vibrator,” Sensors and Actuators 83 (2000) 67-75). Also known are PZT microcantilevers, fabricated using a sol gel method, having a thickness of 4.15 um and sensitivity of 0.44 fC nm−1, which is three times the sensitivity of ZnO microcantilevers.
Due to differences in microstructure and mechanical and thermal stresses at the film-substrate interface, the PZT microcantilever, which is capable of cyclic dynamic imaging, has a dielectric constant of only 850, piezoelectric constant of -27 pm/V and sensitivity of 0.44 fC nm−1 (T. Itoh, et al., “Sol-gel derived PZT force sensor for scanning force microscopy,” Materials Chemistry and Physics 44 (1996) 25-29; Lee, Chengkuo et al., “Self-excited piezoelectric PZT microcantilevers for dynamic SFM-with inherent sensing and actuating capabilities,” Sensors and Actuators A72 (1999) 179-188.)
Researchers have also been investigating the concept of incorporating electrical insulation in a microcantilever to prevent liquid damping. U.S. Patent Publication no. 2005/0112621 discloses an insulation layer surrounding a PZT microcantilever having a thin piezoelectric film in order to prevent conduction in liquid media (See e.g. col. 4, lines 28-36).
These piezoelectric in-solution cantilevers however lack sufficient sensitivity for many AFM applications. Therefore, there remains a significant need to develop highly piezoelectric AFM microcantilevers capable of in-solution detection, operating in both an alternating current mode and a direct current mode and having a femtogram or better sensitivity.